Southern California -- this just in

California tsunami could come with no warning

March 16, 2011 | 8:30
am

Although Southern California is at little risk of a tsunami like the one that devastated Japan, even smaller waves could be damaging, and they could come without warning.

Unlike Japan, California does not have a subduction zone -- a fault where one plate slides under another in an earthquake -- off its coast. The thrusting motion under the sea was what generated the 40-foot tsunami seen in Japan.

But Southern California could see a significant tsunami caused either by a large earthquake off Alaska or by undersea landslides spurred by smaller earthquakes off California. Northern California is at greater risk because of the Cascadia subduction zone, which runs along the Pacific Northwest coast.

Quakes off Alaska and the Pacific Northwest could create 15-foot waves in Southern California and 25-foot tsunamis in the northern part of the state, said California State Geologist John Parrish.

And tsunamis caused by underwater landslides off Southern California could reach as high as 40 feet, although they would be localized and quick to dissipate, said Costas Synolakis, director of the Tsunami Research Center at USC.

That type of event is only expected to strike once in 2,000 or 3,000 years. But, as has happened in Japan, experts say all predictions may go out the window.

“Mother Nature is notorious for not obeying rules that we make,” Parrish said.

A quake off Alaska in 1964 sent 20-foot waves crashing down on Crescent City, Calif., killing 11 people. The same temblor created surges that tore dozens of boats from their moorings and sunk three in the ports of Los Angeles and Long Beach.

USC researchers estimate that a tsunami created by an offshore quake could cost the region $7 billion to $40 billion from port closures alone.

A quake off Alaska would give California six to nine hours lead time to clear the beaches before a tsunami struck, Parrish said. A temblor off the California-Oregon border, on the other hand, might give Northern California towns less than half an hour to prepare.

“That’s not very much time," he said, "especially if it’s 2 o’clock in the morning and you’re trying to wake up a whole town of people and get them up the hill.”

A tsunami generated by an offshore quake and underwater landslide might come with virtually no warning. Southern California, unlike Northern California, has no tsunami measuring instruments off its coast because the area is considered to be low risk.

Maps: Map show potential water inundation during Tsunami in Southern California. Credit: California Department of Conservation. Click on the maps for more detail. Photo: Damage from waves in Crescent City, Calif. Credit: Associated Press